Telecommunications networks transport signals between user equipment at diverse locations. A telecommunications network includes a number of components. For example, a telecommunications network typically includes a number of switching elements that provide selective routing of signals between network elements. Additionally, telecommunications networks include communication media, e.g., twisted pair, fiber optic cable, coaxial cable or the like that transport the signals between switches. Further, some telecommunications networks include access networks.
For purposes of this specification, the term access network means a portion of a telecommunication network, e.g., the public switched telephone network (PSTN), that allows subscriber equipment or devices to connect to a core network. For example, an access network is the cable plant and equipment normally located in a central office or outside plant cabinets that directly provides service interface to subscribers in a service area. The access network provides the interface between the subscriber service end points and the communication network that provides the given service. An access network typically includes a number of network elements. A network element is a facility or the equipment in the access network that provides the service interfaces for the provisioned telecommunication services. A network element may be a stand-alone device or may be distributed among a number of devices.
There are a number of conventional forms for access networks. For example, the digital loop carrier is an early form of access network. The conventional digital loop carrier transported signals to and from subscriber equipment using two network elements. At the core network side, a central office terminal is provided. The central office terminal is connected to the remote terminal over a high-speed digital link, e.g., a number of T1 lines or other appropriate high-speed digital transport medium. The remote terminal of the digital loop carrier typically connects to the subscriber over a conventional twisted pair drop.
The remote terminal of a digital loop carrier is often deployed deep in the customer service area. The remote terminal typically has line cards and other electronic circuits that need power to operate properly. In some applications, the remote terminal is powered locally. In some networks, the remote terminal is fed power over a line from the central office. This is referred to as line feeding or line powering and can be accomplished through use of an AC or a DC source. Thus, if local power fails, the remote terminal still functions because it is typically powered over the line using a battery-backed power source. This allows the remote terminal to offer critical functions like lifeline plain old-fashioned telephone service (POTS) even during a power outage.
Over time, the variety of services offered over telecommunications networks has changed. Originally, the telecommunications networks were designed to carry narrowband, voice traffic. More recently, the networks have been modified to offer broadband services. These broadband services include services such as digital subscriber line (DSL) services. DSL (or xDSL) is a generic name for a family of digital lines such as Asymmetrical Digital Subscriber Line (ADSL), High Bit Rate Digital Subscriber Line (HDSL) and G.SHDSL. Circuits have been designed to incorporate the xDSL, POTS and line power in a twisted pair drop. In the networks that use line powering, the line power in the twisted pair drop has to be monitored and adjusted to ensure a desired amount of current is being applied. A typical method of monitoring the current in the twisted pair drop is by splicing a sense resistor into a rail of the twisted pair drop. An op amp is then coupled across the sense resistor to provide a signal indicative of the current in the twisted pair drop. An additional power source must be provided to power the op amp. A problem with this arrangement is that if a short occurs in the circuit (an output short), the resistor will fail (burn up). To protect against shorts, a protection circuit is then added to protect the sense resistor. The cost and complexity of the sensing circuit, the additional power source for the op amp and the protection circuit is prohibitive. Moreover, prior sensing systems typically have to be coupled to a DC point in the network system which limits the flexibility of the system. It is desired in the art for an improved method of sensing the current flowing through a twisted pair drop without the disadvantages stated above.